No abstract

The effects of sensitization on the metastable pitting behaviour of 304 stainless steel in 3.5 wt.% NaCl solution were investigated via electrochemical measurements and microscopic observations. Results showed that sensitization decreased pitting potential and slightly affected metastable pitting potential. Sensitization treatment increased the nucleation frequency of metastable pitting, promoted its propagation and changed the shape of current transients. The results of scanning electron microscopy and energy-dispersive X-ray spectroscopy showed that metastable pitting was initiated at MnS inclusions for the solution-treated and sensitized samples. The combined effects of the passive film and inclusions were considered to play a vital role on the metastable pitting behaviour.

A remarkable improvement in the pitting corrosion resistance of 304 stainless steel was attempted using a novel duplex passivation treatment method. First, chemical passivation in nitric acid followed electrochemical passivation via potential polarization of step cycling in sodium nitrate electrolyte. Compared with traditional chemical passivation, breakdown potential was increased from 0.31 V SCE to positive than 0.9 V SCE at 70°C in a solution bearing 0.6 M [Cl − ] concentration. The critical pitting temperature was enhanced from 21.5°C to above 70°C in a solution with 6 M [Cl − ] concentration. Impedance analysis and X-ray photoelectron spectroscopy results show that a more compact passive film with a higher ratio of chromium oxide on iron oxide was achieved by electrochemical passivation compared with chemical passivation. Morphology observation suggested that the potential polarization of step cycling slightly increased the dissolution of inclusions after being subjected to chemical passivation. The probable reason for the improvement on pitting resistance is discussed in detail based on inclusion dissolution and the protectiveness in passive film. K E Y W O R D Schemical passivation, electrochemical passivation, pitting corrosion, sodium nitrate, stainless steel

The metastable pitting behavior of austenitic stainless steel (304 SS) and duplex stainless steel (2205 DSS) under U-bending deformation is comparatively investigated. This study highlights the effects of differences in inclusions and matrix between 304 SS and 2205 DSS with regard to the metastable pitting behavior under tensile and compressive deformation. The results show that the pit initiation frequency of 304 SS increases in the order: tensile < asreceived < compressive; in contrast, the initiation frequency of 2205 DSS increases in the order: as-received < tensile < compressive. In addition, tensile and compressive deformation change the preferential initiation sites of metastable pitting in 2205 DSS. It is proposed that the types of transient signals can be comprehensively and objectively distinguished by the two parameters (t g /t rep ) and [(I p /Q g ):(I p /Q rep )]. The correlation relationship between transient signals and pitting morphologies is clarified by cluster analysis. Both tensile and compressive deformations promote the growth of a few metastable pits, thus increasing the transition probability from metastability to stability. Overall, both tensile and compressive deformations are detrimental to the pitting corrosion resistance of the experimental steels.